The invention relates to a lighting device with an optical waveguide plate which comprises a light emission surface and a plurality of channels for accommodating each at least one substantially linear light source, said device being designed in particular for use as a backlight in liquid crystal displays such as LCD picture screens or for use as a planar light radiator.
It is known that LCD picture screens require a backlighting of their entire surface area which is as homogeneous as possible for rendering a picture visible. The difficulty often arises, however, in particular in the case of large lighting devices, that a high luminous intensity cannot be generated with sufficient homogeneity on the entire light emission surface in front of which the picture screen is positioned. This may lead to unpleasant picture effects. Furthermore, these lighting devices should have as small a thickness as possible in many cases.
In principle, two kinds of these lighting devices are distinguished. In the case of so-called direct-lit devices such as known, for example, from JP-5-27238, the light sources, which are usually cylindrical cold- or hot-cathode lamps, are arranged directly behind the picture screen in the optical waveguide plate. The lighting device is then provided with a reflecting layer on its side facing away from the picture screen. To achieve an illumination of the picture screen which is as homogeneous as possible, the distance between the light sources on the one hand and the picture screen on the other hand must not be too small, because otherwise the light radiated directly onto the picture screen by the lamps cannot be compensated for. A comparatively homogeneous light distribution over the picture screen, however, can also be achieved by means of light-scattering layers in front of the picture screen. This requires in general a constructional depth which is more than twice the lamp diameter. A further disadvantage follows from the fact that the light-scattering layers lead to losses, so that the efficiency of such backlight systems (i.e. the proportion of the light generated by the light sources which is actually available for illuminating an LCD picture screen) is at most approximately 50%.
In the case of indirect or side-lit backlighting systems as known, for example, from EP-0717236, the light sources are present at the lateral (narrow) sides of an optical waveguide plate. The light enters the optical waveguide plate through these lateral surfaces and is propagated therein through total reflection against the lateral surfaces of the plate. The light is subsequently coupled out towards the picture screen by means of suitable extraction elements arranged at the front or rear side of the plate. The advantages of this arrangement are that the constructional depth is smaller and that the illumination is usually more homogeneous than in the case of a direct-lit system. The disadvantages are, however, that the total quantity of light is comparatively limited because only the four lateral surfaces are available for introducing the light. In this case, too, it is difficult to achieve a homogeneous illumination through suitable dimensioning of the emission (coupling-out) structures, in particular in the case of larger plates.
It is accordingly an object of the invention to provide a lighting system of the kind mentioned in the opening paragraph which is suitable in particular for use as a backlight for large LCD picture screens and which makes available a homogeneous and intensive illumination of the picture screen in combination with a small constructional depth.
According to claim 1, this object is achieved by means of a lighting device with an optical waveguide plate which comprises a light emission surface and a plurality of channels for accommodating each at least one substantially linear light source, and which is characterized in that said channels are covered with a first reflecting layer at their upper sides facing the light emission surface, and the coupling of the light into the optical waveguide plate takes place through side walls of the channels.
This solution combines the advantages of direct and indirect backlighting systems and accordingly makes available a higher luminous intensity in combination with a homogeneous distribution and a high efficiency of the light sources used. On the one hand, the constructional depth need not be greater than in known indirect backlight systems, because the light sources can be incorporated into the plate. Very flat lighting devices can accordingly be manufactured whose constructional depth does not exceed twice the lamp diameter, i.e. approximately 6 to 8 mm.
On the other hand, an at least equally high luminous intensity can be achieved at the light emission surface as in the case of direct-lit systems, because the number of the light sources is not limited by the number of lateral surfaces. A desired luminous intensity may be achieved through a suitable choice of the number of light sources or channels.
The dependent claims relate to advantageous further embodiments of the invention. A particularly high homogeneity of the light on the light emission surface is achieved in the some embodiments, because it is impossible for any portion of the light issuing from the light sources to reach the light emission surface directly.
The efficiency of the light sources is further enhanced with some embodiments while other embodiments are particularly easy to manufacture.